This invention relates to a network that has a plurality of hubs connected by lines and a method for use in reducing a fault in such a network.
Spanning tree routing, which is ruled in IEEE 802.1d, is used for backup of lines in a LAN. According to the routing, a line that is currently establishing a connection is switched to another line that is reserved.
Conventionally, the spanning tree routing is applied to connections between hubs in a LAN in which each one of hubs is connected with two other hubs. In this LAN, the connections between hubs are strong against faults. However, in this LAN, a hub and a terminal are connected through a single line and therefore, the connections between hubs and terminals are weak against faults.
In another conventional LAN, as shown in FIG. 1, the spanning tree routing is applied to connections between hubs and terminals. In this LAN, hubs 3, 4, 5 and 6 are sequentially connected via a single line. The hubs 3 and 4 are connected via a line 101. The hubs 4 and 5 are connected via a line 102. The hubs 5 and 6 are connected via a line 103. Each of terminals 7, 8, 9 and 10 has two LAN ports each of that is connected to different hub from each other. The terminals 7 and 8 are connected to the hub 3 via lines 111 and 113, and are connected to the hub 5 via lines 112 and 114, respectively. The terminals 9 and 10 are connected to the hub 4 via lines 115 and 117, and are connected to the hub 6 via lines 116 and 118, respectively. The terminals 7, 8, 9 and 10 are operable for the spanning tree routing.
Doubled lines establish connections between hubs and terminals in the LAN, and therefore, the connections are strong against faults. However, in this LAN, connections between hubs are not doubled and consequently are weak against faults. Further, even if the terminals have a function for the spanning tree routing, a fault on the hub might not be detected.
Turning again to FIG. 1, for example, it is assumed that: a fault occurs on the hub 4; and current lines of the terminals 7, 8, 9 and 10 are the lines 111, 113, 115 and 117, respectively. In this case, the terminals 9 and 10 switch the lines 115 and 117 to 116 and 118, respectively. On the other hand, the terminals 7 and 8 keep the lines 111 and 112 because the hubs 3 and 5 are normally working. Consequently, the terminals 7 and 8 can not communicate to the terminals 9 and 10.
The problem can be solved by an upper application program that works on the terminal to constantly confirm connections from the terminal through the other terminals and to switch from the current line to the reserve line when a fault occurs on the LAN. However, the upper application constantly sends packets for confirming connections and, as a result, the packets always occupy all of the connections, and it may take long time to switch from the current line to the reserve line.
Furthermore, in the conventional LAN shown in FIG. 1, each power source of the hubs has to be doubled in order to make the LAN strong against faults on power source.